Uso of Cyclodextrin for Reducing Yellowing of Pulp and Paper

ABSTRACT

The present invention relates the problem of color formation during paper production resulting in yellowing/brightness reversion of the pulp or the paper. This yellowing/brightness reversion can be reduced by the method of the present invention, which method relates to reducing yellowing of pulp and paper comprising adding or generating cyclodextrin to the pulp before or during washing.

FIELD OF THE INVENTION

The present invention relates to a method for preventing or reducing yellowing/brightness reversion of paper and pulps.

BACKGROUND OF THE INVENTION

Paper production involves, as a first step, the formation of pulp from wood. In a conventional kraft pulping procedure, raw or pre-treated wood chips are “cooked” under high pressure and temperature in the presence of alkali and sulfide to dissolve the polymeric lignaceous material between the individual wood fibers. The liberated fibers still contain significant amounts of the colored lignaceous material within the fiber wall that can be removed to varying degrees via a spectrum of conventional bleaching operations/chemistries. Contingent upon the wood species, the pulping conditions and the bleaching method, the bleached wood fiber may contain various amounts of residual lignin and extractives (e.g. resin acids, fatty acids, triglycerides, terpenes, terpenoids, waxes, etc.). Paper and pulps prepared from bleached fiber containing trace amounts of residual lignin and/or extractives have been associated with enhanced sensitivity to color/brightness reversion upon exposure to elevated temperatures. Such thermal extremes can be encountered during conventional shipping and storage operations or within the dryer section and at the paper reel winder as the bleached fiber is converted into the final paper end-product.

At these temperatures, heat-sensitive precursors of chromophores, such as lignin, extractives, as well as oxidation, condensation and cyclization products of carbohydrate origin (e.g. 2-furfural, hydroxymethyl furaldehyde, etc.) can be converted to chromophores resulting in yellowing (color reversion) or brightness reversion of the end product which is undesirable. Therefore such chromophore precursors should preferably be removed from the pulp at washing stages throughout pulping, bleaching and papermaking operations. Currently, a more aggressive washing regime is required to remove such color-forming components and is accomplished by means of investment in new equipment and/or existing equipment upgrades. Not only does this generally correspond to increased water and energy consumption, additional washing may increase the yield loss. Another means to reduce the concentration of chromophores precursors is to incorporate an additional stage into the bleaching sequence which not only necessitates significant capital expenditure, but also requires the increased usage of chemicals. Regardless of the treatment, certain color-forming materials may still remain within the washed pulp (especially those of carbohydrate origin).

SUMMARY OF THE INVENTION

It has now been found that undesirable chromophore precursors, entrapped within cyclodextrin complexes, can be removed during the washing stage of pulp formation.

The present invention therefore relates to a method for reducing or preventing yellowing/brightness reversion of pulp and paper during conditions associated with conventional use (e.g. storage, shipping, paper production, archiving, etc.) comprising entrapping the chromophore precursors present in the pulp within cyclodextrin rings and removing the resulting inclusion complex, such as, during conventional washing by adding or generating the cyclodextrin before or during washing.

The aspect of the present invention therefore relates to a method for reducing yellowing/brightness reversion of pulp and paper comprising direct addition of cyclodextrin to the pulp or in situ generation of cyclodextrin followed by a subsequent washing step to remove the formed cyclodextrin inclusion complexes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to paper production and to the reduction of yellowing and/or brightness reversion of the pulp or the paper product.

A first step in the production of paper is pulp formation. Pulp can be prepared from wood (virgin pulp) in a conventional kraft pulping procedure as described above or from recycled paper. Frequently, pulp will be comprised of a combination of the two fibre sources.

For the purposes of the present invention, any type of paper making process is relevant and/or any paper making pulp can be applied.

In particular embodiments, the amount of pulp based on recycled paper relative to the total amount of pulp is at least 2%, or at least 4%, or at least 6%, or at least 8%, or at least 10%, or at least 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 or at least 95%.

The part of the pulp which is not derived from recycled paper may be derived from mechanical pulping, chemical pulping, and any mixtures thereof, such as, chemi-mechanical pulping, thermo-mechanical pulping, chemi-thermo-mechanical pulping, etc.

For preparing a pulp from a material comprising recycled (or waste) paper, also known as secondary fibres, the material is mixed, dispersed, or suspended in water. This is the process known as pulping. Thereby at least part of the ink and other contaminants such as glue, adhesives, coatings etc. is released from the fibres. A papermaking pulp often comprises both recycled paper and fresh so-called virgin pulp.

The source of recycled fibre can be any of the grades of recycled furnishes known in the art or mixtures thereof, as well as mixtures of recycled fibres with virgin fibres. Major grades of recycled fibre furnishes are for instance MOW (mixed office waste), SOW (sorted office waste), ONP (old newsprint), OMG (old magazines) and OCC (old corrugated containers).

During pulp formation, raw or pre-treated wood chips are “cooked” under high pressure and temperature in the presence of alkali and sulfide to dissolve the polymeric lignaceous material between the individual wood fibers. The liberated fibers still contain significant amounts of the colored lignaceous material within the fiber wall that can be removed to varying degrees via a spectrum of conventional bleaching operations/chemistries. Contingent upon the wood species, the pulping conditions and the bleaching method, the bleached wood fiber may contain various amounts of residual lignin and extractives (e.g. resin acids, fatty acids, triglycerides, terpenes, terpenoids, waxes, etc.).

Extractives such as the hydrophobic fatty acids and esters are very problematic during paper making process. They can cause severe deposits on the process equipment. In addition, the presence of these extractive compounds has been associated with increased sensitivity to thermo-yellowing. Even after lipase treatment, the newly formed long chain fatty acids are still too hydrophobic to dissolve in the process water.

Suspected heat-sensitive precursors of chromophore molecules within conventionally pulped and bleached wood furnishes include residual lignin and extractives as well as oxidation, condensation and cyclization products of carbohydrate origin (e.g. 2-furfural, hydroxymethyl furaldehyde, etc.). Formation of chromophores, in response to elevated temperatures, ultimately manifests as a “yellowing” or “brightness reversion” of the end-product. In most instances, chromophore precursors are lipophilic with limited solubility within the process water.

In the present context, a chromophore comprises any compound which will give rise to a coloring of the pulp or paper. Particularly, the chromophore can be formed from a chromophore precursor at elevated temperatures.

Cyclodextrin may effectively entrap the chromophore and/or chromophore precursor and enhance their solubility in water. This corresponds to enhanced removal of the chromophore/chromophore precursors from the pulp at the washing stage. In addition, many of the precursors adversely affect certain physical properties of paper (i.e. tear, tensile, bulk, etc.) but physically disrupting interfiber interactions through adsorption onto the fiber surface. Enhanced removal through formation of complexes with cyclodextrins may enable a degree of recovery of any number of these affected properties.

The most important feature of cyclodextrins is their ability to form solid inclusion complexes (host-guest complexes). The lipophilic cavity of cyclodextrin molecules provides a micro-environment into which an appropriately sized non-polar moiety can enter to form an inclusion complex. Inclusion in cyclodextrins exerts a profound effect on physiochemical properties of guest molecules as they are temporarily locked or caged within the host cavity, which give rise to some unique beneficial modifications of the guest molecules. The properties include solubility enhancement of highly insoluble guests, stabilization of labile guests against the degradation, masking off flavours, and controlled release of drugs and flavours. In the present invention especially the solubility enhancement of entrapped chromophore/chromophore precursor molecules is utilized.

Cyclodextrin may effectively entrap those hydrophobic materials and enhance their solubility in water. Cyclodextrin can also work with lipases and other enzymes to improve the efficiency.

The paper making process according to the present invention comprises the following steps:

a) preparing a pulp from wood and/or material comprising recycled paper;

b) adding or in situ generating cyclodextrin;

c) washing;

d) making paper from the treated pulp.

Over and above steps a)-d) further, optional, steps may be included, for example one or more of the following steps:

e) treating the pulp with one or more enzymes;

f) de-inking, e.g. by pulping the fibres in the presence of an aqueous alkaline solution, optionally containing a peroxide compound, such as hydrogen peroxide;

g) separation of the fibres from the contaminants, e.g. by screening (coarse and/or fine);

h) centrifugal cleaning;

i) flotation, e.g. using one or more surfactants;

j) washing, e.g. using one or more surfactants;

k) dispersion; e.g. using one or more surfactants; and/or

l) inactivation of the enzymes, if required, e.g. by a heat treatment step.

Any number of these additional steps may be included, and the sequence need not be as indicated a-b-c-d-e-f-g-h-i-j-k-l. The enzyme may be introduced prior to the pulping, during a pulping stage, during or before, preferably right before, a stock preparation stage, or after a flotation or a de-inking stage.

A purpose of the present invention is therefore to provide a method for reducing yellowing of pulp and paper comprising adding to the pulp or in situ generating cyclodextrin followed by a subsequent washing step to remove the formed cyclodextrin inclusion complexes from the pulp. The formation of the cyclodextrin inclusion complexes may also disrupt the conversion of the entrapped chromophore precursors.

Cyclodextrin may be used in the form of alpha-, beta-, and gamma-cyclodextrin.

In one particular embodiment the cyclodextrin is alpha-cyclodextrin.

Cyclodextrin should preferably be present during washing of the pulp but can be added at an earlier stage. In one embodiment cyclodextrin is added during pulp formation. In another embodiment cyclodextrin is added after pulp formation but before washing. In still another embodiment cyclodextrin is added during washing.

In case the chromophore precursor is a fatty acid the ratio of fatty acid to cyclodextrin is in one embodiment in the range from 10:1 to 1:100, particularly from 5:1 to 1:75, even more particularly from 2:1 to 1:50.

Instead of adding cyclodextrin it could also be generated in situ, e.g. generated in the pulp from starch and glycosyltransferase (CGTase).

In one embodiment therefore the cyclodextrin is generated in situ from starch and glycosyltransferase.

Other types of enzymatic treatments may be employed as are well known in the art of paper making and include enzymes selected from the following group of enzymes: proteases, amylases, pullulanases, lipases, hemicellulases, endoglucanases, cutinases, and pectinases; as well as any combination thereof.

The present invention therefore relates to the combination of enzymatic treatment of pulp with addition of cyclodextrins. Suitable enzymes are enzymes that will facilitate the release and complex formation of the chromophore or chromophore precursors.

Xylanase is known in the art as an enzyme capable of reducing yellowing of pulp and paper. In a particular embodiment of the invention the cyclodextrin treatment of the pulp is combined with a xylanase treatment.

EXAMPLES Example 1 Improved Thermo Stability of Bleached Eucalypt Kraft Pulp (BEKP) by Addition of Alpha-Cyclodextrin

Procedure:

1. (6) 1.2 L, stainless steel Launder-Ometer beakers (The Launder-Ometer (Atlas Electric Devices Company, Chicago, Ill., USA) is a standard piece of equipment used in the textile industry that spins the beakers, end-over-end, within a pre-heated, water-filled chamber) containing 400 g of 5% w/w consistency BEKP were prepared (the initial pH of the pulp was not adjusted prior to cyclodextrin application). Pulp LOM Consistency Temp Time BEAKER ID/dose (w/w %) (° C.) (min) pH_(pre) 1 Control 5 55 120 4.1 2 0.01 w/w % 5 55 120 4.1 alpha-cyclodextrin 3  1.0 w/w % 5 55 120 4.1 alpha-cyclodextrin

-   -   2. alpha-cyclodextrin was aliquoted to each beaker according to         the chart above to achieve the desired final concentrations.     -   3. The beakers were then incubated under constant agitation in         the Launder-Ometer for 120 minutes at 55° C.     -   4. After incubation, the beaker contents were pH'd and then         diluted to 4 L (0.5% consistency)     -   5. The pulp slurry was then stirred for 2 minutes     -   6. (3) 2 odg handsheets were prepared from each pulp, couched         according to TAPPI standard procedure and placed between two dry         blotter pads (chromed plates were not used during this         particular handsheet preparation).     -   7. Sheets were dried overnight under TAPPI conditions.     -   8. To simulate a more aggressive washing stage, the remainder of         the pulp following initial handsheet formation, was subjected to         washing (10 L DI H₂O) within a dynamic drainage jar equipped         with a 125P screen. The pH of the washed pulp should approach         neutrality).     -   9. (3) 2 odg handsheets were then prepared from each washed pulp         and couched and dried as before.     -   10. All handsheets were analyzed for ISO brightness, CIE &         Hunter Whiteness Index, E313 and DIN6167 Yellowness Index and         the CIE color space coordinates, L, a* and b* with a Technidyne         ColorTouch PC (Technidyne Corporation, New Albany, Ind.).     -   11. The handsheets were then “aged” for 15 minutes at 160° C.         and the optical qualitative measurements repeated.

The results of the study are given in Table 1 below and show that the thermo stability of BEKP was improved resulting in a reduction in yellowing of pulp pads by the addition of alpha-cyclodextrin. TABLE 1 Percentage of yellowing reduction and brightness loss after accelerated aging compared to the controls before and after washing. Treatment No wash Washed % reduction in E313 Yellowness Index relative to untreated control Control 0.00 0.00 0.01% alpha-cyclodextrin 1.47 9.72  1.0% alpha-cyclodextrin 5.35 6.36 % reduction in ISO brightness loss relative to untreated control Control 0.00 0.00 0.01% alpha-cyclodextrin 0.00 1.07  1.0% alpha-cyclodextrin 0.51 0.87 % reduction in CIE b* relative to untreated control Control 0.00 0.00 0.01% alpha-cyclodextrin 1.42 9.88  1.0% alpha-cyclodextrin 5.35 6.23

Example 2 Removal of Fatty Acids from Solution by Alpha-Cyclodextrin

Removal of fatty acids from solutions by addition of alpha-cyclodextrin was tested by removal of linoleic acid. Linoleic acid is the most abundant fatty acid in wood extractives and constitutes a potential chromophore. A 0.1% linoleic acid suspension and a 1.0% alpha-cyclodextrin solution was prepared as follows:

1. 250 microliters linoleic acid was transferred to a 250 ml volumetric flask.

2. Deionized water was slowly added while agitating on a whiny mixer.

3. Linoleic acid solution stays suspended as long as agitation is continued.

A 1.0% alpha-cyclodextrin solution was prepared by dissolving 2 g of alpha-cyclodextrinin 200 ml deionized water.

The 0.1% linoleic acid suspension was mixed with the 1.0% cyclodextrin solution at different ratios of fatty acid to cyclodextrin and the increase in turbidity was determined. The results are given in the Table 2 below. TABLE 2 Turbidity (NTU, Nephelometric Turbidity Unit) measurements of mixtures of fatty acid (FA) and cyclodextrin (CD). Fatty acid Volume Cyclodextrin, Volume Water Ratio Turbidity Test tube 0.1% (ml) 1.0% (ml) (ml) FA:CD (NTU) 1 Linoleic 0 — 0 30 — 0 acid 2 Linoleic 20 Alpha 1 9 2:1  64 acid 3 Linoleic 20 Alpha 10 0 1:5  168 acid  3b Linoleic 20 — 0 10 — 72 acid 4 Linoleic 10 Alpha 10 10 1:10 252 acid 5 Linoleic 10 Alpha 20 0 1:20 378 acid  5b Linoleic 10 — 0 20 — 40 acid 6 Linoleic 6 Alpha 24 0 1:40 414 acid  6b Linoleic 6 — 0 24 — 24 acid 7 Linoleic 4 Alpha 20 6 1:50 342 acid  7b Linoleic 4 — 0 26 — 20 acid

An increase in the turbidity reflects precipitation of the entrapped FA.

The results show that it is possible to remove fatty acids, exemplified by linoleic acid, from solution by entrapment in cyclodextrin. 

1-7. (canceled)
 8. A method for reducing yellowing of pulp and paper comprising adding to the pulp cyclodextrin or generating a cyclodextrin in the pulp before or during washing of the pulp.
 9. The method according to claim 8, wherein the cyclodextrin comprises a α-cyclodextrin.
 10. The method according to claim 8, wherein the cyclodextrin comprises a β-cyclodextrin.
 11. The method according to claim 8, wherein the cyclodextrin comprises a γ-cyclodextrin.
 12. The method according to claim 8, wherein the cyclodextrin is provided by enzymatic generation of cyclodextrin in the pulp.
 13. The method according to claim 11, wherein the enzyme comprises CGTase.
 14. The method according to claim 8, wherein the method further comprises adding an xylanase to the pulp. 